Patent application title: BAND-PASS CURRENT MODE CONTROL SCHEME FOR SWITCHING POWER CONVERTERS WITH HIGHER-ORDER OUTPUT FILTERS

Abstract:

A DC-DC converter is described that contains multiple estimators and is
self-oscillation. The converter also contains at least a fourth order
output filter. The converter contains both feedback and feed-forward
paths. The estimators estimate the current through inductors in the
filter by sensing the voltage across the inductors.
The forward feed path contains a comparator. The self-oscillation is
provided by hysteresis in the comparator or by a phase-shift network
connected to the comparator. The estimators comprise extra windings
coupled to each inductor or a series combination of a resistor and a
capacitor connected in parallel with the inductor.

Claims:

1. A switch-mode power converter comprising:an output filter of at least
fourth order containing a plurality of inductors and capacitors; anda
control system comprising a plurality of inductor estimators positioned
to sense voltage across the inductors and estimate current through the
inductors, a feedback loop providing the estimates to an input of the
control system, and a self-oscillation mechanism to provide
self-oscillation of the control system, wherein the output filter is
connected with an output of the control system and comprises a plurality
of low pass filters connected in series.

2. The switch-mode power converter of claim 1, wherein the control system
comprises a forward feed path containing an OpAmp, a
proportional-integral (PI) compensator, and a comparator, the PI
compensator connected between the OpAmp and the comparator in the forward
feed path.

3. The switch-mode power converter of claim 2, wherein the comparator
contains hysteresis and is connected such that the self-oscillation
mechanism comprises the comparator.

4. The switch-mode power converter of claim 2, wherein the control system
further comprises a phase-shift network connected between the OpAmp and
comparator, the self-oscillation mechanism comprising the phase-shift
network and the comparator.

5. The switch-mode power converter of claim 2, further comprising second
feedback loops each connecting one of the capacitors with an inverting
input of the OpAmp, one of the second feedback loops containing a
resistor and another of the second feedback loops containing a parallel
combination of a capacitor and another resistor.

6. The switch-mode power converter of claim 2, wherein the feedback loop
comprises a gain disposed between each inductor estimator and the input
of the control system, each gain being independently controllable.

7. The switch-mode power converter of claim 2, wherein each inductor
estimator comprises extra windings coupled to the respective inductor.

8. The switch-mode power converter of claim 7, wherein the feedback loop
comprises a low pass filter and the extra windings are connected in
series between ground and the low pass filter.

9. The switch-mode power converter of claim 2, wherein each inductor
estimator comprises a series combination of a resistor and a capacitor
connected in parallel with the respective inductor.

10. The switch-mode power converter of claim 9, wherein each inductor
estimator further comprises a differential amplifier whose inputs are
connected to either side of the respective capacitor.

11. The switch-mode power converter of claim 2, wherein the control system
further comprises a driver driving power transistors connected in a
push-pull configuration, the driver and power transistors connected
between the OpAmp and the output filter.

12. A method of providing power conversion comprising:providing a feed
forward path;providing self-oscillation along the feed forward path;low
pass filtering a self-oscillated signal using an output filter of at
least fourth order;estimating current through inductors in the output
filter by sensing voltages of the inductors; andfeeding back the
estimates along a feedback loop to the feed forward path.

13. The method of claim 12, further comprising providing a differential
amplification and proportional-integral (PI) compensation along the
forward feed path.

15. The method of claim 12, wherein providing the self-oscillation
comprises providing a controllable phase-shift network connected between
a differential amplifier and a comparator in the feed forward path.

16. The method of claim 12, further comprising providing independently
controllable gain for each inductor current along the feedback loop.

17. The method of claim 12, wherein each estimation is provided using
extra windings coupled to the respective inductor.

18. The method of claim 12, wherein each estimation is provided using a
series combination of a resistor and a capacitor connected in parallel
with the respective inductor.

Description:

TECHNICAL FIELD

[0001]The present application relates to power amplifiers. In particular,
the application relates to the power amplifiers having output inductors
and feedback loops containing estimates of the current through the
inductors.

BACKGROUND

[0002]Power amplifiers are used in a variety of applications. In
communication systems, for example, power amplifiers provide the desired
signal strength for radio frequency (RF) wireless transmissions between a
base station and a wireless handset. A power supply provides power to the
power amplifier. In some communication power amplifier systems, an
Ultra-Fast Tracking Power Supply (UFTPS) is employed to better control
the power amplifier in the transmitter to provide the desired
instantaneous output power level, thereby better maximizing the
efficiency of the power amplifier by limiting the wasted power.

[0003]To realize power savings in the system, the UFTPS is to act as a
low-dissipation controllable voltage source from DC to the RF bandwidth.
It is thus desirable for the UFTPS to have sufficiently low power losses.
The UFTPS typically employs a switch-mode power converter to provide such
a low power loss. Besides power savings, reducing the response time is
also desirable. Accordingly, it is desirable for the output voltage of
the UFTPS to respond relatively quickly to changes in the reference
voltages--ideally at a rate equivalent to the bandwidth of the
transmitted signal (e.g., 25-150 kHz). Further, to avoid interference
when intermodulation of the output occurs with the transmitted RF signal,
it is desirable for the output ripple voltage of the switch-mode power
converters to be relatively small, e.g., 5-50 mVpp. Also, it is desirable
for the UFTPS to have a relatively low output impedance (e.g., 10-100
mΩ) from DC to the RF bandwidth.

[0004]One example of a commonly-used power supply is a single-phase buck
converter (in which a single DC-DC converter is disposed between the
input and the load). A buck converter in a UFTPS application may contain
one or more output filters coupled with multiple proportional-derivative
(PD) control loops to form a proportional-integral-derivative (PID)
controller. However, while a buck converter that contains multiple LC
filters and control loops is useful in an UFTPS application, the
components used in the control loops are subject to practical
implementation problems such as increased power consumption, noise
sensitivity, and sensitivity to circuit parasitics. It is accordingly
desirable to provide a buck converter that reduces these problems.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005]Embodiments will now be described by way of example with reference
to the accompanying drawings, in which:

[0006]FIG. 1 illustrates a BPCM control buck converter according to one
embodiment.

[0007]FIG. 2 illustrates a BPCM control buck converter according to
another embodiment.

[0008]FIG. 3 shows a portion of a BPCM control buck converter according to
another embodiment.

[0009]FIG. 4 shows a portion of a BPCM control buck converter according to
another embodiment.

[0010]FIG. 5 illustrates a BPCM control buck converter according to
another embodiment.

[0011]FIG. 6 illustrates one embodiment of a power amplifier system
containing a UFTPS.

DETAILED DESCRIPTION

[0012]A power amplifier system containing a buck converter and method of
providing DC-DC conversion is described. The power amplifier system
contains multiple estimators and self-oscillating control in a multiple
order filtered buck converter. The buck converter contains feedback and
feed-forward control paths. The feed-forward path contains a comparator.
The self-oscillation is provided either by a phase-shift network disposed
before the comparator in the feed-forward signal path or by hysteresis in
the comparator. The estimators estimate currents through the inductors
and capacitors in the output filter by sensing the voltage across the
inductors.

[0013]The various individual components are well-known to one of skill in
the art and will not be described in detail. Further, other circuitry
that is associated with the power amplifier system and is well-known to
one of skill in the art will not be described for conciseness.

[0014]FIG. 1 illustrates one embodiment of a switch-mode power converter.
The power converter shown is a Band Pass Current Mode (BPCM) controlled
buck converter, with Band-Pass Current Mode Feedback arranged as to
achieve the dynamics of a Global Loop Integrating Modulator (GLIM). The
BPCM buck converter in general contains a switching power converter with
an output filter in which multiple loops provide feedback from the output
filters. The BPCM buck converter 100 shown uses an estimate of the
inductor current rather than using direct voltage measurements as
feedback to control the buck converter 100. The BPCM current feedback
signals, along with the feedback of one of the capacitor voltages
(Vcl) of a capacitor C1 in the output filter, function together
to effectively create feedback control of the voltage and current of this
capacitor C1, thereby realizing proportional-derivative (PD)
feedback control of the voltage of the capacitor C1 without
employing a differentiating element for the differentiating portion of
the PD loop coupled with the capacitor C1. Avoiding the use of a
differentiator reduces problems due to noise in the physical controller
implementation.

[0015]Additionally, the BPCM buck converter 100 contains a
proportional-integral-derivative (PID) controller, which is a control
loop feedback mechanism that corrects an error between the output voltage
and a desired voltage by calculating an error correction and then
providing the error correction to adjust the output voltage. The PID
controller contains proportional, integral, and derivative paths. The
first of these paths determines the reaction to the current error, the
second determines the reaction based on the sum of recent errors, and the
third determines the reaction to the rate at which the error has been
changing. A weighted sum of these paths is used to adjust the output
voltage.

[0016]As shown, the buck converter 100 contains an output voltage
controller 102 to which a negative reference voltage -VREF is
supplied through a low pass filter. The output voltage controller 102 is
connected with an adder 104. The output of the adder 104 is connected
with an input of a compensator 106. The compensator 106 may contain, for
example, a proportional-integral (PI) feed forward in which the
derivative path is eliminated, which, combined with the PD emulation of
the BPCM feedback, leads to control solution functionally equivalent to
that of a PID controller but with added immunity to circuit parasitics
and switching noise. The output of the compensator 106 is connected with
an input of a comparator 108 with hysteresis, which may be a conventional
Schmitt trigger. The hysteresis of the comparator 108 provides a
self-oscillation mechanism. The output of the comparator 108 alternates
between the positive and negative supply voltages dependent on whether
the voltage supplied to the positive terminal is larger than the voltage
supplied to the negative voltage (which, as shown, is ground) or
vice-versa.

[0017]The output of the comparator 108 is connected to a driver 110 to
activate the driver 110, which in turn drives a pair of power transistors
112 connected in a simple push-pull configuration (although other
configurations can be used). The transistors 112 can be field effect
devices, such as MOSFETs, or bipolar devices, such as BJTs.

[0018]The outputs of the transistors 112 are connected to a first LC
output filter 114. The first LC output filter 114 is connected in series
with a second LC output filter 116. The first and second LC output
filters form a 4th order output filter, which decrease the ripple of
the output voltage from the buck converter 100. The first and second LC
output filters 114, 116 contain an inductor L1, L2 and a
capacitor C1, C2, respectively. The switching frequencies of
the transistors 112, which are much higher than the maximum frequency
response of the human ear (about 20 kHz), cause radio-frequency
interference. The output filters 114, 116 reduce this interference and
allow the output signal to correspond to the input signal. However, as
the output filters 114, 116 are potentially undamped and the current
drawn from the output terminal (node Vout) is time-varying, the
output voltage Vout is controlled via negative feedback. Multiple
feedback loop are used because of the difficulty in compensating for the
phase lag of the entire fourth order filter in a single control loop. The
voltage from the second LC output filter 116 (the output voltage
VOUT) is integrated and supplied as feedback through an outer PD
control loop to an operational amplifier (OpAmp) in the controller 102,
where the difference between the reference signal VREF and the
output voltage VOUT is used to adjust the voltage from the
controller 102. The voltage from the first LC output filter 114 is
supplied as feedback through an inner PD control loop to the adder 104
such that this voltage is subtracted from the voltage from the controller
102.

[0019]Rather than the currents in the output filters being directly
supplied to an integrator to thereby provide PD feedback, these currents
are estimated. Specifically, the current through the capacitor C1
(and the current through the inductor L1) in the first LC output
filter 114 is estimated by sensing the voltage across the inductor
L1 in the first LC output filter 114 and then integrating or
low-pass filtering the sensed result. The current through the capacitor
C2 (and the current through the inductor L2) in the second LC
output filter 116 is similarly estimated by sensing the voltage through
the inductor L2 in the second LC output filter 116. As the voltages
through the inductors L1, L2 are relatively large, they may be
sensed relatively easily. The estimate of the current through the
capacitor C1 in the first LC output filter 114 and the estimate of
the current through the capacitor C2 in the second LC output filter
116 are supplied to the adder 104 through first and second gains 118,
120, respectively. The first and second gains 118, 120 may be the same or
different and are either preset (i.e., unchangeable once implemented) or
controllable as desired. The amplified estimates are subtracted by the
adder 104 so that the difference between the amplified estimates is added
to the output of the controller 102 and the voltage of the capacitor
C1 of the first LC output filter 114 subtracted therefrom.

[0020]The resulting signals from the output filters 114, 116 emulate PD
current feedback without the use of noise- and parasitic-sensitive
differentiation of the capacitor voltages--in FIG. 1, only a single
capacitor voltage is supplied directly as feedback to the output voltage
controller 102. Instead, the inductor voltages in the LC output filters
are sensed and the currents through the capacitors estimated and supplied
as the PD feedback.

[0021]The manner in which the signals travel through the converter 100 is
now described. Specifically, the OpAmp in the controller 102 receives a
sine wave input. An integrator connected between the input and output of
the controller 102 integrates the difference between input and output
voltages of the OpAmp, resulting in the triangular waveform. The
comparator 108 receives the triangular waveform, modified by the adder
104 and compensator 106 and generates square voltage pulses. These pulses
are then amplified by the transistors 110 and transmitted to the output
filters 112, 114 to reconstruct the desired output signal VOUT. Note
that switching of the comparator 108 at high speed results in a square
wave whose pulse width and frequency is dependent on the input voltage
and frequency and whose average value corresponds to the buck converter
input.

[0022]FIG. 2 illustrates another embodiment of a BPCM control buck
converter. Similar to the embodiment of FIG. 1, the BPCM buck converter
200 of FIG. 2 contains an output voltage controller 202 to which a
reference voltage is supplied, an adder 204, a compensator 206, a
comparator 208 with hysteresis, a driver 210, transistors 212, and first
and second LC output filters 214, 216. These elements are connected
together in a manner similar to that of FIG. 1. Further, similar to the
embodiment of FIG. 1, the voltages in the inductors L1, L2 in
each of the first and second LC output filters 214, 216 are sensed and
estimates are made of the current through the capacitor C1, C2.
Specifically, in the embodiment of FIG. 2, the voltages in the inductors
L1, L2 are sensed by extra windings. The current through the
capacitor C1 in the first LC output filter 214 is estimated using
floating sense windings as a difference block. This voltage difference,
which corresponds to the derivative of the capacitor C1 current, is
then integrated using a low pass filter Rest, Cest disposed in
the feedback path between the first LC output filter 214 and the adder
204. Using this inner PD control loop permits a wide variety of outer PD
control loops to be added.

[0023]One example of an output voltage controller 300 is illustrated in
FIG. 3. As shown, the output voltage from the capacitor C2 of the
second LC output filter 216 as shown in FIG. 2 is connected to the
inverting terminal of the OpAmp in the controller 300 through a parallel
resistor/capacitor RP2, CD2 combination. The low-pass filtered
voltage difference output between the inductors L1, L2 is
connected to the inverting terminal of the OpAmp through a resistor
Rcfb1. The capacitor C1 is also connected to the inverting
terminal of the OpAmp through a resistor RPI to convert the current
to a voltage. Feedback is supplied between the output and the inverting
terminal of the OpAmp through another integrator of a series
resistor/capacitor RPI, CPI combination.

[0024]In another example of a controller and circuitry connected thereto
is illustrated in FIG. 4, the voltage controller 400 is similar to that
of FIG. 3. Unlike the embodiment of FIG. 3, in which a
hysteresis-containing comparator is used to provide the self-oscillation,
a phase-shift network provides the self oscillation by providing a phase
shift to the output triangular wave from the OpAmp. Thus, rather than the
output of the OpAmp of the voltage controller being connected directly to
the input of a hysteresis-containing comparator, as shown in the
embodiment of FIG. 4, the phase-shift network 404 is disposed between the
output of the OpAmp of the voltage controller 400 and the input of a
comparator 402. The comparator 402 of FIG. 4 does not contain hysteresis
as the self oscillation is provided by the phase shift network. The
phase-shift can either be preset or controllable as desired, with only
the preset version being shown. Other implementations of a phase-shift
network using different topologies may be used as desired.

[0025]FIG. 5 illustrates another embodiment of a buck converter. This buck
converter 500 contains a controller 502, a compensator 504 with
hysteresis, a driver 510, a pair of push-pull transistors 512, and first
and second LC output filters 514, 516 again connected in a manner similar
to that of the buck converter 100 of FIGS. 1 and 2. Unlike the embodiment
of FIG. 2, the buck converter 500 of FIG. 5 does not sense the voltage of
the inductors using extra coils, which may be relatively large, bulky,
and expensive. Instead, a series combination of an RC filter is connected
in parallel with the inductor L1, L2 in each of the LC output
filters 514, 516. The voltage across the capacitor Cest1, Cest2
in each of the series LC combinations provides the input to a
differential amplifier D1, D2. The output of each differential
amplifier D1, D2 is connected to the inverting terminal of the
OpAmp through a respective resistor Rcfb1, Rcfb2.

[0026]Although only one type of filter is shown in the figures, filters
with other characteristics and orders may be used. Each of these filters
may contain an inductor of which the voltage thereacross is detected and
the current estimated rather than being directly provided in a feedback
loop. The components in the forward and reverse portion of the loop may
be altered to achieve the desired loop characteristics.

[0027]FIG. 6 illustrates a power amplifier system 600. The power amplifier
system 600 contains a baseband modulator 602 whose output is connected to
the inputs of both a UFTPS module 604 and a power amplifier module 606.
The UFTPS module 604 contains a buck converter similar to that of FIGS.
1-5 and provides the power supply for the power amplifier module 606. As
shown, the output voltage of the baseband modulator 602 is combined with
a DC bias voltage and then amplified by a power transistor in the power
amplifier module 606. The output of the UFTPS module 604 is provided as a
supply voltage to the power transistor through an inductor. The output of
the power amplifier module 606 is supplied to a high pass filter 608,
whose output is provided as the output of the power amplifier system 600.

[0028]Note that although the embodiments shown in the figures contain
multiple current estimators, in other embodiments at least one direct
connection can be used and at least one current estimator can be used
when multiple LC filters are used.

[0029]The buck converters and power amplifier described herein are useful
in narrowband RF systems with variable RF amplitude. Such systems include
Tetra (TErrestrial Trunked RAdio), Tetra2, iDen (Integrated Digital
Enhanced Network) systems. The buck converters can be used in multiple
communication applications including individual handsets and other
subscriber applications or base stations.

[0030]It will be understood that the terms and expressions used herein
have the ordinary meaning as is accorded to such terms and expressions
with respect to their corresponding respective areas of inquiry and study
except where specific meanings have otherwise been set forth herein.
Relational terms such as first and second and the like may be used solely
to distinguish one entity or action from another entity or action without
necessarily requiring or implying any actual such relationship or order
between such entities or actions. The terms "comprises," "comprising," or
any other variation thereof, are intended to cover a non-exclusive
inclusion, such that a process, method, article, or apparatus that
comprises a list of elements does not include only those elements but may
include other elements not expressly listed or inherent to such process,
method, article, or apparatus. An element proceeded by "comprises . . .
a" does not, without more constraints, preclude the existence of
additional identical elements in the process, method, article, or
apparatus that comprises the element.

[0031]Those skilled in the art will recognize that a wide variety of
modifications, alterations, and combinations can be made with respect to
the above described embodiments without departing from the spirit and
scope of the invention defined by the claims, and that such
modifications, alterations, and combinations are to be viewed as being
within the purview of the inventive concept. Thus, the specification and
figures are to be regarded in an illustrative rather than a restrictive
sense, and all such modifications are intended to be included within the
scope of present invention. The benefits, advantages, solutions to
problems, and any element(s) that may cause any benefit, advantage, or
solution to occur or become more pronounced are not to be construed as a
critical, required, or essential features or elements of any or all the
claims. The invention is defined solely by the appended claims including
any amendments made during the pendency of this application and all
equivalents of those claims as issued.